1. Field of the Invention
The present invention relates to a piezoelectric transformer-inverter for converting dc current to ac current using a piezoelectric transformer.
2. Description of the Related Art
In recent years, a liquid crystal display provided with a backlight has generally been used as a display device in portable data processing equipment, such as a portable video camera and a laptop computer. A fluorescent light tube such as a cold-cathode tube is used as a light source for the backlight. A high-voltage ac current must be applied in order to ignite the fluorescent light tube, and a combination of a battery and an ac adapter is generally used as an input power source for the portable data processing equipment such as a laptop computer. For this reason, a fluorescent light tube igniting device such as a dc/ac inverter is required in order to convert low-voltage dc current, supplied from the input power source, to high-voltage ac current which can be used to ignite the fluorescent light tube. Recently, there has been progress in the development of a piezoelectric transformer-inverter using a piezoelectric transformer, which is comparatively smaller than an electromagnetic transformer, as this kind of fluorescent light tube igniting device.
When attempting to adjust the brightness of the backlight in such a piezoelectric transformer-inverter, it is necessary to control the voltage and current output to the fluorescent light tube to desired values. In a conventional piezoelectric transformer-inverter, methods of stabilizing the voltage and current output to the fluorescent light tube include such methods as: (1) changing the output voltage to the fluorescent light tube by changing the drive frequency of the piezoelectric transformer, using the drive frequency dependency of the boost ratio of the piezoelectric transformer; and (2) using a dc-dc converter and a PWM (pulse-width modulation) inverter circuit to convert the input power voltage from a desired voltage value to an average voltage value, and applying this via a piezoelectric transformer drive circuit to the primary electrodes of the piezoelectric transformer, thereby changing the (average) output voltage to the fluorescent light tube.
FIG. 1 shows an example of a piezoelectric transformer-inverter 11 using these methods. The constitution of this piezoelectric transformer-inverter 11 will be explained based on the circuit of FIG. 1. The piezoelectric transformer-inverter 11 comprises a piezoelectric transformer 13 for applying a voltage to a fluorescent light tube 12, a frequency controller 14 for controlling the drive frequency of the piezoelectric transformer 13 and keeping the current flowing to the fluorescent light tube 12 at a predetermined value, a booster (piezoelectric transformer drive circuit) 15 for dividing the frequency of the signal output from the frequency controller 14, and applying a drive voltage of the divided signal to the primary electrodes of the piezoelectric transformer 13, and a drive voltage controller 16 for performing PWM control to ensure that the drive voltage applied to the piezoelectric transformer 13 has a predetermined average value.
The frequency controller 14 comprises a current voltage converter 1, a rectifier 2, a comparator 3, an integrator 4 and a VCO (voltage-controlled oscillator) 5. In this frequency controller 14, the current voltage converter 1 converts the current flowing through the fluorescent light tube 12 to a voltage signal, the rectifier 2 converts this voltage signal to a dc detector voltage; then, the comparator 3 compares this with a reference voltage Vref, and when the detector signal voltage is lower than the reference voltage Vref, the comparator 3 outputs a high-level signal to the integrator 4. While receiving the high-level signal, the integrator 4 lowers its output voltage by a fixed ratio. The VCO 5 outputs a square pulse of a frequency proportional to the control voltage which is input from the integrator 4, and the piezoelectric transformer 13 is driven via the booster 15 at the frequency of the VCO 5.
The booster 15 has a push-pull operation using two transistors 18 and 19 and two coils 20 and 21. The VCO 5 outputs a square pulse of a frequency which is twice the drive frequency of the piezoelectric transformer 13, the booster 15 uses a divider 10 to divide the frequency of the signal output from the VCO 5 of the frequency controller 14 to a half frequency, and the transistors 18 and 19 are alternately switched ON and OFF in accordance with the signal output from the divider 10. When the transistors 18 and 19 switch ON, current flows from the input power source (not shown) to the coils 20 and 21, charging them with electromagnetic energy, and when the transistors 18 and 19 turn OFF, the electromagnetic energy charged in the coils is released, and a sine wave voltage higher than the input power voltage V.sub.DD is applied to the primary electrodes of the piezoelectric transformer 13.
Since the piezoelectric transformer 13 is driven in a higher region than the resonant frequency, when the current flowing to the fluorescent light tube 12 drops, the frequency of the VCO 5 also drops, increasing the boost ratio of the piezoelectric transformer 13; conversely, when the current flowing to the fluorescent light tube 12 increases, the frequency of the VCO 5 rises, decreasing the boost ratio of the piezoelectric transformer 13, and as a result, the boost ratio of the piezoelectric transformer 13 is controlled so that a stable current flows to the fluorescent light tube 12.
The drive voltage controller 16 comprises a switching transistor 6, a flywheel diode 7, a comparator 8 and a rectifier 9. The VCO 5 of the frequency controller 14 outputs a triangular wave signal at a frequency which is twice the drive frequency of the piezoelectric transformer 13 to the comparator 8. The rectifier 9 smoothes the drain voltage of the transistor 18 of the booster 15 and inputs the smoothed drain voltage to the comparator 8. The comparator 8 compares the triangular wave signal voltage from the VCO 5 with the rectifier output and controls the switching transistor 6 to adjust the duty ratio of the voltage V.sub.DD supplied from the input power source (not shown) to the booster 15, and keeps the drive voltage of the piezoelectric transformer 13 fixed. Consequently, the drive voltage controller 16 controls the drive voltage applied to the piezoelectric transformer 13 so that it is stabilized at a predetermined average voltage, even when the input power voltage V.sub.DD changes.
However, among light-adjusting systems for controlling output voltage and output current to a fluorescent light tube, a method which uses the drive frequency dependency of the boost ratio of the piezoelectric transformer has a disadvantage that, when the input power voltage has risen, the drive frequency of the piezoelectric transformer must be increased and the piezoelectric transformer must be operated with a small boost ratio, causing the efficiency of the piezoelectric transformer to deteriorate.
To solve this problem, as shown in FIG. 1, with the piezoelectric transformer-inverter 11 in the system for PWM controlling the input power voltage V.sub.DD, the drive voltage is kept at a predetermined average value by controlling the duty ratio using the drive voltage controller 16, and therefore, even when the input power voltage V.sub.DD has a wide range (particularly, when the input power voltage is high), the piezoelectric transformer 13 can be operated close to the resonant frequency and the efficiency of the piezoelectric transformer 13 does not substantially deteriorate.
According to this piezoelectric transformer-inverter 11, the drive voltage controller 16 controls the drive voltage to a predetermined average value, and the frequency controller 14 modulates the light by changing the piezoelectric transformer drive frequency. When lowering the tube current, the frequency controller 14 controls the drive current so that the boost ratio of the piezoelectric transformer 13 decreases, lowering the tube current, whereby the frequency controller 14 outputs a higher drive frequency and the piezoelectric transformer 13 operates at a frequency of poor efficiency far from the resonant frequency. As a consequence, the piezoelectric transformer-inverter 11 has the disadvantage that, when a small current flows to the fluorescent light tube 12, the piezoelectric transformer 13 operates at a frequency of poor efficiency, lowering the overall efficiency of the piezoelectric transformer-inverter.